In general, pistons used in internal combustion engines such as diesel engines and gasoline engines are made from aluminum alloys or cast iron, and when the engine is running, temperatures at the top face of the crown (the piston head) reach approximately 300° C. with aluminum pistons, and approximately 500° C. with cast iron pistons. Furthermore, ring grooves formed in the pistons for mounting piston rings also reach high temperatures when the engine is running, and in order to prevent adverse effects on the function of the piston rings, it is necessary to limit this temperature rise. For this reason, an annular cavity is commonly provided in this area for circulating a cooling fluid such as oil.
Such annular cavities can be cast using a readily disintegrating core, which is made from sand or the like, in order to form the annular cavity; but the work of molding the core, as well as disintegrating the core and removing it from the cast body after casting, is difficult, time-consuming and expensive. Furthermore, the formation of an annular cavity, by separately casting the top land and the skirt of the main piston body, and then joining the top land and the skirt at the dividing surfaces thereof, by pressure welding or the like, is known, for example, from Japanese unexamined patent application JP-2001-107803-A.
This prior method has the advantage of allowing for efficient production of the main piston body, as compared to single molded products. However, with methods such as that described in JP-2001-107803-A, when the top land and the skirt, which are two separate parts, are pressure welded, a curled scale forms at the edge of the pressure welding faces. The size of this scale is proportional to the amount of pressure applied, and if this curled scale grows very large, the volume of the annular cavity formed at the interior of the contact faces will be reduced in direct proportion to the volume of the curled scale, which adversely affects the flow characteristics of the cooling fluid, and may lead to burning of the piston head, or result in knock due to overheating the piston head.
The present invention is a reflection of the situation described above, and an object thereof is to provide a method for efficiently manufacturing a high quality internal combustion engine piston having an annular cavity allowing for good circulation of cooling fluid.